This application proposes a programmatic effort aimed at elucidating the immunological barrier to cardiac xenotransplantation and the development of strategies to overcome that barrier. The rationale for this effort is a very critical shortage of human organs available for allotransplantation. The immunological hurdles include hyperacute rejection in unmodified recipients and acute vascular rejection in xenograft recipients who have been unmodified recipients and acute vascular rejection in xenograft recipients who have been depleted of anti-donor antibodies and/or complement. Other hurdles involve graft injury mediated by neutrophils, natural killer cells and lymphocytes. The program uses a preclinical model involving the transplantation of porcine hearts into the cynomolgus monkeys. The contribution to the rejection process of natural antibodies, complement, and inflammatory cells as well as of reperfusion injury, will be addressed. One central theme of the program focuses on endothelial cells as a target of oxidant injury and in humoral and cell mediated responses as an effector system. Another central theme focuses on the acquired resistance of vascularized organ grafts to humoral mediated injury; test the mechanisms which may contribute to this "accommodated" state. Project 1 considers the contribution of oxidant mediated injury of humoral mediated rejection. Project 2 considers the role of natural antibodies and Project 3 considers the role of complement in this process. Project 4 investigates endothelial cell responses to humoral injury. Project 5 explores the contribution of neutrophils to acute vascular rejection. Projects 6 and 7 investigate how natural killer cells and T-lymphocytes, respectively might interact with a xenogeneic organ. Project 8 studies the pathogenesis and potential treatment of humoral rejection in the large animal model. The overall effort of the program is supported by an administrative core and a cell culture and immunopathology core.